Automotive spark-ignited direct-injection gasoline engines

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 521
ratio, which greatly reduces the in-cylinder temperature. As
a result HCs which do not participate in the combustion
process have a much lower probability of post-combustion
oxidation. The exhaust temperature is also low, making it
more difficult for current three-way catalyst technology to
operate at the high efficiencies necessary for satisfactory
tailpipe emission levels. The low exhaust temperature also
eliminates the possibility of any hydrocarbon oxidation in
the exhaust port, as is observed with the PFI engine operating
under a similar condition [199]. The key considerations
regarding increased UBHC emissions from stratified-charge
GDI engines are:
• flame extinction occurs for the very lean mixtures near
the outer boundary of the stratified charge;
• poor combustion can occur in overly rich regions near the
piston crown or cylinder wall due to spray–wall wetting;
• lower combustion temperature reduces the degree of
post-flame oxidation of UBHC;
• lower exhaust gas temperature degrades the conversion
efficiency of the catalyst system;
• lower exhaust gas temperatures significantly reduce the
occurrence of UBHC oxidation in the exhaust port.
In general, it has been found that direct gasoline injection
can yield a slight increase in UBHC emissions at idle, and
can exhibit substantial increase at part load. For part-load
operation at higher engine speeds, another source of UBHC
is the decrease in the time available for mixture preparation,
which is limited for direct injection. This can result in
diffusion burning at the surface of liquid droplets. The application
of EGR at low engine loads can also increase the
UBHC emissions substantially, and, when coupled with
the use of a higher compression ratio for the GDI application,
can result in additional amounts of UBHC being forced
into crevices, resulting in an incremental increase in the
UBHC emissions [40,223–225].
A number of the very early observations that related to the
UBHC emissions of DISC engines may still be applied
directly to current GDI engines. A range of possible reasons
for the increase in the UBHC emissions for DISC systems at
low load were listed by Balles et al. [271], and by Frank and
Heywood [227] for the TCCS combustion system. It was
noted that at low load the mixture becomes too lean prior to
ignition due to the mixing and diffusion of the injected fuel.
As a result, the combustion efficiency was found to be low
for many cycles, and the associated UBHC emissions were
excessive. Generally, at low load an overall ultra-lean
mixture was supplied in the TCCS engine that may have
resulted in over-mixing. Due to the small quantity of fuel
injected into the combustion chamber, the cyclic variation
of the evaporation rate was found to be a significant factor.
The UBHC emissions were traced mainly to cycles with the
lowest combustion efficiency. Frank and Heywood [227–
230] reported that the UBHC emissions exhibited little
dependence on the piston crown temperature, which
would indicate that the adherence of liquid fuel on the piston
surface is not the main source of the UBHC emissions for
the combustion system tested.
Any delay in the flame kernel development around the
spark gap, or in the propagation rate of the flame through the
mixture, is a contributing factor that increases the UBHC
emissions. Frank and Heywood [228] pointed out that
engine combustion events with large combustion delays
yield a marked increase in the UBHC emissions. This indicates
that, for the stratified combustion of lean mixtures, the
resident time of the fuel inside the combustion chamber
should be as short as possible. Thus, the spray characteristics
at the end of the fuel injection event are important in
determining the UBHC emissions. The fuel injected as the
pintle is closing may not be well atomized, and has the least
amount of time available to vaporize. With many pintle
designs, the last fuel injected may have a trajectory that
differs from that of the main jet [229], resulting in an
increase in UBHC emissions.
Ronald et al. [45] studied the effect of air–fuel ratio on the
UBHC emissions of a single-cylinder research GDI engine.
The UBHC emissions were found to increase continuously
with an enleanment of the air–fuel ratio for the stratifiedcharge
mode. This was attributed to increasingly prevalent
pockets of excessively lean mixture and, as a result, an
incomplete combustion event. A further reason may be a
reduced reaction rate along the exhaust pipe due to the
decrease in the exhaust temperature. As the charge becomes
more stratified, the amount of fuel compressed into the top
land area of the piston decreases significantly, and less fuel
is in contact with the cylinder wall. As a consequence, the
UBHC emissions may be lower when compared with PFI
operation. Although the PFI engine exhibits a marked
decrease in the CO emissions for lean mixtures, the CO
emissions of the GDI engine remain nearly constant.
Iiyama and Muranaka [77] concluded from calculations
that pockets of mixture within the combustible range for
air–fuel ratios richer than 30 are distributed randomly inside
the flame. As a result, the flame does not propagate
uniformly through these regions, yielding an increase in
UBHC emissions. It was reported that high UBHC emissions
at low load is an inherent feature of the stratified combustion
concept itself. To reduce this type of UBHC emission, it is
necessary to generate a highly stratified flow field. This
tends to suppress the formation of rich pockets inside the
combustion zone. One of the technologies for accomplishing
this is the two-stage injection system proposed by Miyamoto
et al. [201] and by Hattori et al. [202].
Anderson et al. [199] conducted a detailed computational
analysis on the UBHC formation process of a stratified
charge GDI engine using the KIVA code. It was found
that there can be as much as 10% of the injected fuel remaining
in liquid form at the time of ignition for late injection.
This was true for both side-mounted and centrally-mounted
injectors. The fuel on the piston crown was found to be the
largest source of UBHC emissions. This impingement of